A fixation device (100) for bones comprises a member (1) which is to be fixed to one or more bones (40, 42) and has at least one bore (2a－c,2',2")for receiving a bone screw (30), wherein the at least one bore comprises a first internal thread portion (22a－c；22'). The bone screw for insertion through the bore into the bone has a first shaft section provided with a first external thread portion (32) arranged to cooperate with the internal thread portion of the at least one bore, and a head section (36) having a diameter larger than that of the shaft section to provide a catch arranged to engage with a stop formed in the bore. The bone screw further has a second shaft section which comprises a clearance groove (34a) extending between the catch of the head section and the external thread (32) of the first shaft section. The clearance groove allows disengagement of the two thread portions, such that the bone screw is prevented from being unscrewed off the bore when it is loosened within the adjacent bone, e.g., a vertebral body. The member (1) may comprise a side wall (4) of a cage used in an intervertebral implant device, or may represent a plate of a bone plate assembly.一種骨骼固定裝置(100)，包括可固定於一或以上骨骼(40,42)之構件(1)，以及供納入骨骼螺釘(30)用之至少一腔孔(2a－c,2',2")，其中至少一腔孔包括第一陰螺紋部(22a－c；22')。供插穿腔孔進入骨骼內用之骨骼螺釘，具有第一軸段，設第一陽螺紋部(32)，配置成與至少一腔孔之陰螺紋部嚙合，和頭段(36)，其直徑大於軸段，形成掣子，配置成與腔孔內形成的止擋部結合。骨骼螺釘又有第二軸段，包括間隙凹溝(34a)，在頭段的掣子和第一軸段的陽螺紋部(32)間延伸。間隙凹溝容許二螺紋部脫離，當骨骼螺釘在鄰近骨骼(例如椎骨本體)鬆動時，可防止其從腔孔脫落。構件(1)可包括椎間植入機件內所用籠體的側壁(4)，或呈骨骼板總成之板。
